Do you have your system on a good surge suppressor or UPS? Did you disconnect all of the external devices so you have just the core system with the keyboard & mouse/trackpad connected and was it still not able to boot up?
One of the things you may need to look at is your buildings wiring. Get a line checker which can check the ground and power. You could have a bad ground service or a miswired outlet. Is the outlet 20 Amp (US) and is the outlets line isolated back to the breaker panel? While I've not encountered it with a Mac Pro I've seen power supplies in systems burnt by power surges, power outages & lightning. Which one good reason to have an expensive system plugged into a good UPS which protects your system from these events.